-use Focus Blast over Hidden Power Fighting.
-the HP investment is unnecessary on Gengar, who is frail. Invest 4 of those HP points into SpAttack. Also...
-You have 10 too many EV points on that. Remove 10 from the HP stat.
-Hypnosis and Dream Eater is bad, as it can be switched away from and Hypnosis is unreliable. Instead, give it any combination of Dazzling Gleam, Sludge Wave, Thunderbolt, and Giga Drain.
Otherwise, it's fine.